This is THE top level container element for an HTML document.
It specifies that the content of the document is text with HTML markup
tags. The content of an HTML document is a HEAD element which contains
the title of the document plus other information ABOUT the
document. Following the HEAD element comes the visible content of the
document in the BODY element. Alternately, an author may specify a
FRAMESET series in place of the BODY element that enumerates the
layout of sub-documents to be contained in the master document.
This is not a required element in the specification, and most
popular browsers will not react oddly to its absence. Still,
this is a basic top level element in HTML document structure and
it is good style to use it.